Five Current Peaks in Voltammograms for Oxidations of Formic Acid

J. Phys. Chem. B 2005, 109, 15659-15666

15659

Five Current Peaks in Voltammograms for Oxidations of Formic Acid, Formaldehyde, and Methanol on Platinum Hiroshi Okamoto,* Wataru Kon, and Yoshiharu Mukouyama Department of Natural Sciences, College of Science and Engineering, Tokyo Denki UniVersity, Hatoyama, Saitama 350-0394, Japan ReceiVed: March 29, 2005; In Final Form: June 24, 2005

We have found that five current peaks are present in the voltammograms in the positive and negative sweep directions for the oxidations of formic acid, formaldehyde, and methanol on Pt in the potential range of 0.05-1.8 V, although the experimental conditions for the peaks to appear are different. In particular, a current peak at ca. 0.6 V, the negative slope of which on the positive side is closely related to autocatalysis, inducing oscillation, has been observed even for methanol. We have clarified that the three substances produce very similar voltammograms at a very slow sweep rate, such as 0.1 mV/s, and show some of the same behaviors of the peaks in their voltammograms. All these facts support the idea that the electrochemical oxidation mechanisms for the three substances have the same dominating elementary reaction steps, which induce oscillation phenomena, although with different reaction and adsorption rate constants.

Introduction acid,1-15

The electrochemical oxidations of formic formaldehyde,3-5,7,16-20 and methanol3-5 on Pt in acidic media all show oscillation behavior, especially potential oscillation under constant current conditions. It is almost established that they all proceed basically via the same reaction mechanism, namely, the dual-path mechanism consisting of direct and indirect paths. Several candidates for the intermediate present in the indirect path, which is poisonous at low potentials, have been proposed, such as CHO (HCO) for methanol oxidation21 and both COH and CO for formic acid,22 formaldehyde,23 and methanol24 oxidations. On the other hand, there have been few discussions on the intermediate in the direct path, which is reactive, and COOH has mainly been thought as a candidate for the formic acid oxidation.22 Recently, however, it has become almost established by infrared spectroscopy that the intermediate in the indirect path is the same for the oxidations of the three substances, being adsorbed carbon monoxide, CO.25-27 It has also been found that the spectroscopically identified intermediate in the direct path for the three reactions is the same, being adsorbed formate, HCOO.28-30 As Parsons et al.31 have pointed out, it seems that the reaction mechanisms for the three substances have the same dominating elementary reaction steps, which then bring about oscillation phenomena. Meanwhile the voltammograms for the oxidations of the three substances appear different. The ordinarily obtained voltammogram for the oxidation of formic acid shows five current peaks, one of which is a shoulder, in the positive and negative sweep directions in the potential range of 0.05-1.8 V, whereas that of formaldehyde appears different with five current peaks including two shoulders and that of methanol appears considerably different with four current peaks including one shoulder. Although it seems comparatively easy to relate the peaks from formaldehyde to those of formic acid, it is difficult to relate the peaks from methanol to those of formic acid or formaldehyde. * To whom correspondence should be addressed. Fax: +81-49-2962960. E-mail: [email protected].

Particularly, the voltammogram only for methanol has not shown a current peak at ca. 0.6 V, the negative slope of which on the positive side is thought to be necessary for the appearance of oscillation, because the negative slope in the voltammogram is the origin of autocatalysis.8,9,20 If the dominating elementary reaction steps are the same for the three substances, it should be possible to relate the peaks present in voltammograms among the substances, and particularly, it should be possible to observe a current peak at ca. 0.6 V clearly. To achieve this, we have made an effort based on the information on the differences in both the catalytic activity and the rate of formation of the adsorbed CO among the substances. By changing the sweep rates, sweep potential ranges, and concentrations of the substances, we finally have been able to prove this point. In this paper, we describe how we have been able to relate the peaks present in the voltammograms among the three substances and to find the conditions to obtain distinct peaks in the voltammogram. Experimental Section The electrochemical measurements were carried out in a conventional three-electrode cell with a Pt net (99.99%, 2.5 cm2) as the working electrode, a normal hydrogen electrode (NHE) as the reference electrode, and a platinum wire separated with a glass frit from the reaction solution as the counter electrode. The solution temperature was 315 K (42 °C). The Pt net was pretreated by heating it in a hydrogen flame for about 10 s and just before each run by repeatedly applying a triangular potential sweep between 0.05 and 1.4 V. The experiments were carried out very carefully to avoid contamination with impurities, because it took almost several tens of minutes and sometimes longer than an hour to finish an experimental run. The supporting electrolytic solution was 0.5 mol/dm3 (M) sulfuric acid (Kanto “Ultrapur” or Wako “Super Special Grade”) diluted with Millipore “Milli-Q” water. The formic acid or methanol solution was prepared by adding formic

10.1021/jp0516036 CCC: $30.25 © 2005 American Chemical Society Published on Web 07/27/2005

15660 J. Phys. Chem. B, Vol. 109, No. 32, 2005

Okamoto et al. were seen in the positive sweep direction: peak I at ca. 0.6 V, peak II at ca. 0.85 V, and peak III at ca. 1.45 V. In the negative sweep direction, two current peaks were seen: peak IV at ca. 0.71 V and peak V at ca. 0.47 V, which appeared as a shoulder. The current peaks are named according to the literature notation.20,33-35 The reason for the appearance of each current peak is briefly explained here. With an increase in the potential, peak I appears due to the oxidation reaction (the direct path):

HCOOH + * f HCOO* + H + e-

(1)

HCOO* f CO2 + H+ + e- + *

(2)

and then due to the increasing adsorption of poisonous species, probably water adsorbed adjacent to the adsorbed CO8:

H2O + CO* + * f H2O*-CO*

(3)

Here * stands for an adsorption site. The overall current of peak I is suppressed by the formation of adsorbed CO:

HCOOH + * f CO* + H2O.

(4)

The negative slope on the positive side of peak I is necessary for the appearance of the potential oscillation, because the oscillation potential range includes the potential range of the negative slope, which induces autocatalysis, crucial for the appearance of oscillation. Peak II appears due to the oxidation of the adsorbed CO (the indirect path): Figure 1. Ordinarily obtained voltammograms for the oxidations of formic acid (a), formaldehyde (b), and methanol (c). Concentrations of each organic substance and H2SO4 are 0.1 mol/dm3 (M) and 0.5 M, respectively. The temperature and sweep rate are 315 K (42 °C) and 100 mV/s. The potential is plotted vs NHE. The solid line is a voltammogram in the potential range of 0.05-1.4 V, and the dashed line is that in the 0.05-1.8 V range.

acid (Aldrich, ACS reagent) or methanol (Wako “Super Special Grade”) to the supporting electrolytic solution. The formaldehyde solution was prepared by dissolving paraformaldehyde (Merck, extrapure) in a warm supporting electrolytic solution. To deoxygenate the electrolytic solution, nitrogen gas (Nippon Sanso, over 99.9999%) was bubbled through the solution before the measurement, and it was flowed over the solution during the measurement. We used a function generator (Hokuto Denko, HB-105) and a potentiostat/galvanostat (Hokuto Denko, HA-501G). The time sequence of the potential or current values was acquired through an AD converter (National Instruments, PCI-6034E) and saved in a personal computer after averaging of 100 values acquired at a sampling rate of 100 kHz or slower. At the same time, it was recorded using an X-T or X-Y recorder. The experimental details are described in previous papers.32,33 Results and Discussion Ordinarily Obtained Voltammograms for Oxidations of the Three Substances. The oxidations of formic acid, formaldehyde, and methanol apparently produced different voltammograms, as shown in Figure 1, under the same experimental conditions in which the concentration of the three substances was 0.1 M, the temperature was 42 °C, and the sweep rate was 100 mV/s. For the oxidation of formic acid, three current peaks

H2O*-CO* f CO2 + 2H+ + 2e- + 2*

(5)

CO* + H2O* f CO2 + 2H+ + 2e- + 2*

(6)

or

producing CO2 and a bare Pt site, where formic acid is oxidized (the direct path) at the same time via reactions 1 and 2, and then due to the surface deactivation brought about by the formation of Pt oxides and/or hydroxides:

H2O + * f OH* + H+ + e-

(7)

H2O + * f O* + 2H+ + 2e-.

(8)

Peak III probably appears due to the oxidation of formic acid on the oxidized surface via reactions 1 and 2 and then due to the deactivation induced by the more oxidized surface via reaction 8 and the following reaction:

OH* f O* + H+ + e-

(9)

With a decrease in the potential, peak IV appears due to the activation of the surface sites through the reduction of platinum oxides and/or hydroxides via the reverse reactions of reactions 7 and 8. Peak V probably appears due to the desorption of the as yet unidentified poisonous species33 or due to surface rearrangement and then due to both the decrease in the velocity of oxidation reactions 1 and 2 and the increase in the velocity of formation of the adsorbed CO via reaction 4. As for the oxidation of formaldehyde, as shown in Figure 1b, the voltammogram showed two current peaks with one

Voltammograms for HCOOH, HCHO, and CH3OH Oxidation

J. Phys. Chem. B, Vol. 109, No. 32, 2005 15661

Figure 2. Voltammograms related to sweep rates for the oxidations of formic acid, formaldehyde, and methanol. Peak I appears at high sweep rates for formic acid, whereas it does not for formaldehyde and methanol. At a sweep rate of 0.1 mV/s all three substances exhibit similar voltammograms.

shoulder in the positive scan direction: a shoulder around 0.75 V, which probably corresponds to peak I in the voltammogram for the formic acid oxidation, peak II at ca. 0.88 V, and peak III at ca. 1.48 V. In the negative scan direction, one shoulder and one peak were seen: a shoulder at ca. 0.78 V, which probably corresponds to peak IV, and peak V at ca. 0.65 V. It is comparatively easy to presume the relation of the peaks and shoulders for the oxidation of formaldehyde to those of formic acid, whereas it is not so easy to presume that between the oxidations of methanol and formic acid. In the voltammogram for the oxidation of methanol, as shown in Figure 1c, there appeared one merging current peak at ca. 0.8 V and peak III at ca. 1.45 V in the positive sweep direction and a single current

peak at ca. 0.73 V in the negative sweep direction. It is difficult to observe peak I, which is necessary for the appearance of oscillation. It is also difficult to assign the last peak at ca. 0.73 V to either peak IV or peak V. A question then arises whether we can relate the peaks and shoulders found in the voltammogram for the oxidation of methanol to those of formic acid. It is noted that Figure 1 shows that the velocity of formation of the adsorbed CO is faster for formaldehyde and methanol than for formic acid, because the former two do not produce a distinct peak I probably due to the near saturation adsorption of CO whereas the latter does. Figure 1 also shows that the rate of oxidation is faster for formic acid and formaldehyde than


Okamoto et al.

Figure 3. Voltammograms related to the lower limit potential, El, for the oxidations of formic acid, formaldehyde, and methanol. Peak I appears for formaldehyde with El ) 0.4 V at a sweep rate of 1 V/s.

for methanol, because the overall current for the former two is greater than for formaldehyde. Because peak III was present as a distinct current peak at a potential similar for the three substances, and because we have been interested especially in the oscillation behavior in the potential range of 0.1-0.8 V, we did not further investigate peak III and, therefore, measured the voltammograms in the range of 0.05-1.4 V thereafter. Peak I for Formaldehyde. Because we believed that the electrochemical oxidations of the three substances proceed via the dual-path mechanism with the same dominating elementary reaction steps, as mentioned in the Introduction, we postulated that it was possible to correlate the peaks and shoulders found among the voltammograms for the oxidations of the three substances. To prove this and to confirm the relationship deduced above for the peaks and shoulders from formic acid and formaldehyde, we first measured the voltammograms at various sweep rates. As seen from the voltammograms for the oxidation of formic acid in Figure 2a1-a5, peak I clearly appeared at a high sweep rate of 1 V/s, while with a decreasing sweep rate it became small and merged with peak II, and finally the merged peak

Figure 4. Voltammograms in the positive or negative sweep direction starting at 0.7 V after the potential is held at Eh for 100 s (a2) and the voltammogram with Eh ) 0.4 V in the negative sweep followed by the positive sweep (b2) for 0.1 M methanol. (a1) and (b1) are the potential profiles for (a2) and (b2). The dotted line curve is a stationary voltammogram at 100 mV/s.



Figure 5. Voltammograms related to the lower limit potential, El, for the oxidations of 0.1, 1, and 3 M methanol. Peak I appears for methanol at a concentration of 1 or 3 M, El ) 0.2-0.4 V, and a sweep rate of 3 V/s. The single current peak in the negative sweep direction shows a slight split with 1 or 3 M methanol and El ) 0.05 V at a sweep rate of 0.01 V/s.

shifted to a more negative potential. The clearly shown peak IV at a high sweep rate decreased to a trace, and finally, only peak V remained in the negative sweep direction at the same potential as that of the merged peak in the positive sweep direction. Namely, in the 0.05-1.4 V potential range there remained a single current peak in both sweep directions, which has been described previously29 in detail. For the voltammograms describing the oxidation of formaldehyde, as shown in Figure 2b1-b5, a clearly separated peak I was not observed throughout the sweep rates examined, although a small peak appeared on the negative side of peak II at 10 mV/s. We do not think that the small peak corresponds to peak I of formic acid, because for formic acid, as shown in Figure 2a1-a5, with a decreasing sweep rate peak I appears before the merging of peaks I and II, while for formaldehyde the small peak appears after the merging. We postulate by comparison of the voltammogram changes for formic acid and formaldehyde that the small peak is one of the peaks which the merged peak temporarily splits to form with a decreasing sweep rate. Regarding peak IV, it was not clear at 1 V/s, while it

became an obvious shoulder at 0.1 V/s and then became small with a decreasing sweep rate. The voltammogram at the lowest sweep rate shown in Figure 2b5 was very similar to the one for formic acid shown in Figure 2a5, in that they both showed a single current peak in both the positive and negative sweep directions at almost the same potential in the potential range of 0.05-1.4 V except for the skirts of peak III. As for the voltammograms describing the oxidation of methanol, as shown in Figure 2c1-c5, a clear peak I was not observed for all the sweep rates examined, while a shoulder attached to peak II seen at 1 V/s showed a peak merging similar to that of formic acid with a decreasing sweep rate. The single current peak present in the negative sweep direction did not even show the slightest peak division for all the sweep rates examined. It is noteworthy that at a sweep rate of 0.1 mV/s there exists a single current peak in both sweep directions at almost the same potential over a wide potential range in the same way as for formic acid and formaldehyde, although a small difference in the peak potential is present among them. This finding


Okamoto et al.

Figure 6. Reversibility of peak I for the forward and backward potential sweeps for formic acid (a2), formaldehyde (b2), and methanol (b3). (a1) is the potential profile for (a2), and (b1) is that for (b2) and (b3). The dotted line curve is a stationary voltammogram at 100 mV/s.

suggests that the single current peak in the negative sweep direction for methanol oxidation is peak V at a sweep rate of 0.1 mV/s, although it is still obscure whether the single current peak corresponds to peak IV or V at sweep rates higher than 0.1 mV/s. Incidentally, Figure 2 shows that the descending order of the ease of obtaining the voltammogram with the single current peak in both sweep directions over a wide potential range is methanol, formaldehyde, and then formic acid. To find a clearly separated peak I in the voltammograms for the oxidation of formaldehyde and methanol, and to find a separation of the peak present in the negative sweep direction for methanol, no matter how small the separation was, we then measured the voltammograms by increasing the lower limit potential, El, of the sweep range. This is because formaldehyde and methanol form the adsorbed CO more rapidly than formic acid and the adsorbed CO is formed faster at a low potential than at a high potential. As a result, for the oxidation of formic acid as a reference, as shown by the solid curves in Figure 3a1-a4, peak I was clearly observed with El higher than 0.05 V and less than 0.4 V, at which peak I began to merge with peak II. The result shows that, with a decreasing El, namely, with an increasing amount of the adsorbed CO, the originally probably one current peak divides into two peaks, peaks I and II, and then peak I becomes small, which is consistent with our previous observation.8 Furthermore, as shown by the dashed curve in Figure 3a1, when the sweep rate was high, such as 1 V/s, peak I was also clearly seen even with El ) 0.05 V. The result is consistent with the fact that the adsorbed CO is formed at a low potential, because the sweep at a fast sweep rate experiences low potentials for such a short time that the amount of the adsorbed CO is small. For formaldehyde oxidation, as shown by the solid curves in Figure 3b1-b4, peak I was not observed despite increasing El at a sweep rate of 0.1 V/s. However, as shown by the dashed curves in Figure 3b1-b4, at a sweep rate of 1 V/s peak I appeared at El ) 0.2 V and became distinct by increasing El to 0.4 V. Thus, we can say that a clearly separated peak I is observed only when the sweep rate is high, such as 1 V/s, and El is high, such as 0.4 V, at the same time. As for methanol, as shown in Figure 3c1-c4, however, peak I was not observed with an increasing sweep rate or El, or with both of them increasing at the same time. Incidentally, no separation of the

peak present in the negative sweep direction was observed with El and the sweep rates shown in the figure. Peaks I, IV, and V for Methanol. To explore peak I for methanol, we paid attention to the shoulder attached to peak II shown in Figure 1c. We then measured the voltammograms in the positive or negative sweep direction from 0.7 V, around which the shoulder existed, just after holding the potential at Eh for 100 s, as shown in Figure 4a1. As a result, as shown in Figure 4a2, at Eh ) 0.4 and 0.6 V two clearly separated peaks were observed, namely, one in the negative direction and the other in the positive direction, whereas at Eh ) 0.8 V only a single current peak appeared. It may be possible that the current peak at a lower potential found in the negative direction is assigned to peak I, because it appears after the potential is held at a low Eh, where the adsorbed CO forms, and because the appearance of peak I requires the adsorbed CO.8 The peak, however, did not show a reversibility,8,20 one of the characteristics of peak I, for the forward and backward potential sweeps, as shown in Figure 4b2. Consequently, we made further efforts to find an unquestionable peak I. Because methanol is less reactive than formic acid and formaldehyde, as mentioned earlier, we then increased the concentration of methanol from 0.1 to 1 or 3 M to increase the oxidation velocity, although there was a concern in increasing the velocity of forming the adsorbed CO. Figure 5 shows the result with the systematically changed concentration, sweep rate, and El. When the concentration was 0.1 M, as shown in Figure 5a1-a4, peak I was not observed even when the sweep rate was as high as 3 V/s and El was 0.4 V at the same time. When the concentration was 1 M, as shown in Figure 5b1-b4, peak I appeared for the first time and became distinct with an increasing El at a sweep rate of 3 V/s, whereas it was not observed at a sweep rate of 1 V/s for El shown in the figure. When the concentration was 3 M, as shown in Figure 5c1-c4, peak I was more easily seen. As shown later, the current peak was reversible for the forward and backward sweeps. Thus, we conclude that the oxidation of methanol also produces a clear peak I when the methanol concentration is high, such as 1 or 3 M, the sweep rate is high, such as 3 V/s, and El is high, such as 0.2-0.4 V. We suppose that the key to obtaining a distinct peak I for the oxidation of methanol is to find a balance between the rate of oxidation and the rate of formation of the adsorbed CO. If



Figure 8. Peak II current, I(peak II), related to the holding time, t, defined in Figure 7a. The data are from Figure 7. The peak II current becomes independent of t with increasing t for all three substances.

Figure 7. Voltammograms starting at potential Es after the potential is held at Eh for various times, t, for formic acid (b), formaldehyde (c), and methanol (d). (a) is the potential profile. All three substances show that with increasing t the originally probably one current peak is divided into two peaks, peaks I and II, and peak I becomes distinct and then becomes small and disappears. At the same time the height of peak II becomes independent of t. The dotted line curve is a stationary voltammogram at 100 mV/s.

the oxidation rate is low, the concentration should be high, while if the rate of formation of the adsorbed CO is high, the reverse is true. In the case of methanol, we found that we had to increase the concentration together with a high sweep rate and El to find the balance to obtain peak I. In addition to this, as shown by the dotted-dashed curves in Figure 5a1,b1,c1, the single current peak present in the negative sweep direction observed for 0.1 M methanol produced a shoulder on the negative side of the original peak for 1 or 3 M at a sweep rate of 0.01 V/s. Thus, we can say that the single current peak present in the negative sweep direction for 0.1 M methanol is peak IV. And peak V appears when the methanol concentration is high, such as 1 or 3 M, and the sweep rate is low, such as 0.01 V/s, or when the concentration is 0.1 M and the sweep rate is very low, such as 0.1 mV/s, as shown before. Thus, we conclude that the oxidations of all three substances

produce five current peaks, peaks I-V, in their voltammograms, if the experimental conditions are carefully controlled. Other Voltammogram Characteristics Common to the Three Substances. As previously described,8,20 we found that peak I is reversible for the forward and backward potential sweeps. Figure 6a1,a2,b1,b2 shows the reversibility of peak I for the oxidations of formic acid and formaldehyde as a reference. The change in the current on the positive side of peak I was almost reversible for the back-and-forth sweeps except when a large amount of the adsorbed CO was present due to a long holding time, t, at 0.2 V, as shown in Figure 6a2. For formaldehyde the sweep rate was high, such as 1 V/s, to prevent additional adsorbed CO from forming during the back-and-forth sweeps. The reversibility was also observed for methanol, as shown in Figure 6b3, where the sweep rate was as high as 3 V/s to produce peak I, as found earlier, although the current in the reverse and the following forward sweeps was a little low compared to the one in the first forward sweep. This is probably due to additional formation of the adsorbed CO during the backand-forth sweeps. As previously described,8 we found that for formic acid with an increase in the amount of the adsorbed CO one current peak is first divided into two peaks, peaks I and II, after which peak I becomes distinct and then becomes small and finally disappears, as shown in Figure 7b as a reference. The experiment was carried out as follows: as shown in Figure 7a, the potential was first raised to 1.4 V for 5 s to oxidize and eliminate the adsorbed carbon-containing substances and then was kept at 0.4 V, where the adsorbed CO formed, for time t, and finally a voltammogram was measured starting at 0.5 V, where the adsorbed CO hardly formed in a short time for formic acid. Figure 7b shows that the height of peak II becomes independent of t with increasing t, as also clearly shown in Figure 8. The reason for this is probably as follows: the current in the peak II potential range is the sum of the current via the indirect path due to the oxidation of the adsorbed CO (reaction 5 or 6) and the current via the direct path due to the oxidation of formic acid (reactions 1 and 2) on bare Pt sites just produced by the oxidation of the adsorbed CO, and therefore, when the surface becomes almost saturated with the adsorbed CO, the current sum becomes constant. As shown in Figure 7c,d, formaldehyde and methanol also showed that with increasing t the originally probably one current peak was divided into two peaks and one of the peaks, peak I, became distinct and then became small enough to disappear. The potential at which the voltammogram was started was high, such as 0.65 V, because formaldehyde and methanol form the adsorbed CO more easily than formic acid. For methanol peak II appeared even with a small t. This is probably due to the

15666 J. Phys. Chem. B, Vol. 109, No. 32, 2005 rapid formation of the adsorbed CO during the sweep. The height of peak II also became independent of t with increasing t, as shown in Figure 8. Thus, we conclude that formaldehyde and methanol show a behavior similar to that of formic acid regarding the appearance of peak I and the constant peak current of peak II, both of which are related to the amount of the adsorbed CO. Thus, we have found that all peaks, peaks I-V, are present in the voltammograms for the oxidations of formic acid, formaldehyde, and methanol though under different experimental conditions, and the behavior of the peaks is similar among the three substances. We suppose the reason for different conditions producing five current peaks is that the oxidation velocity in the direct path (reactions 1 and 2) and the formation velocity of adsorbed CO in the indirect path (reaction 4) are different for the three substances, such as the oxidation velocity is higher for formic acid and formaldehyde than for methanol, while the formation velocity of adsorbed CO is higher for formaldehyde and methanol than for formic acid. These facts strongly support the idea that the electrochemical oxidation mechanisms for the three substances have the same dominating elementary reaction steps, which induce oscillation phenomena, although with different reaction and adsorption rate constants. Conclusions We were able to show that formic acid, formaldehyde, and methanol all exhibited five current peaks in the potential range of 0.05-1.8 V in their voltammograms though under different experimental conditions, probably because the oxidation velocity in the direct path of the dual-path mechanism and the formation velocity of adsorbed CO are different among the three substances. In addition to this, we clarified the following: (1) at a very slow sweep rate of 0.1 mV/ s, all three substances produced a similar voltammogram, (2) all the substances showed the same behavior regarding the appearance of peaks I and II related to the amount of the adsorbed CO, such as the reversibility of peak I during the back-and-forth sweeps and the invariance of the peak II height when the Pt surface was almost saturated with the adsorbed CO. All the results strongly support the idea that the electrochemical oxidation mechanisms for the three substances have the same dominating elementary reaction steps, which induce oscillation phenomena, although with different reaction and adsorption rate constants. Acknowledgment. This work was partially supported by the Research Institute for Science and Technology of Tokyo Denki University under Grants Q03M-01 and Q04M-06.

Okamoto et al. References and Notes (1) Mu¨ller, E. Z. Elektrochem. 1923, 29, 264. (2) Wojtowicz, J.; Marincic, N.; Conway, B. E. J. Chem. Phys. 1968, 48, 4333. (3) Horańyi, G.; Inzelt, G.; Szetey, EÄ . J. Electroanal. Chem. 1977, 81, 395. (4) Horańyi, G.; Inzelt, G.; Szetey, EÄ . Acta Chim. Acad. Sci. Hung. 1978, 97, 299. (5) Nova´k, M.; Visy, Cs. Acta Chim. Acad. Sci. Hung. 1980, 105, 47. (6) Anastasijevic, N. A.; Baltruschat, H.; Heitbaum, J. Electroanal. Chem. 1989, 272, 89. (7) Schell, M.; Albahadily F. N.; Safar, J.; Xu, Y. J. Phys. Chem. 1989, 93, 4806. (8) Okamoto, H. Electrochim. Acta 1992, 37, 37. (9) Strasser, P.; Lu¨bke, M.; Raspel, F.; Eiswirth, M.; Ertl, G. J. Chem. Phys. 1997, 107, 979. (10) Anastasijevic, N. A.; Baltruschat, H.; Heitbaum, J. J. Electroanal. Chem. 1989, 272, 89. (11) Inzelt, G.; Kerte´sz, V. Electrochim. Acta 1993, 38, 2385. (12) Inzelt, G.; Kerte´sz, V. Electrochim. Acta 1997, 42, 229. (13) Kerte´sz, V.; Inzelt, G.; Barbero, C.; Ko¨tz, R.; Haas, O. J. Electroanal. Chem. 1995, 392, 91. (14) Lańg, G. G.; Ueno, K.; U Ä jva´ri, M.; Seo, M. J. Phys. Chem. B 2000, 104, 2785. (15) Naour, C. Le; Moisy, Ph.; Le´ger, J. M.; Petrier, C.; Madic, C. J. Electroanal. Chem. 2001, 501, 215. (16) Buck, R. P.; Griffith, L. R. J. Electrochem. Soc. 1962, 109, 1005. (17) Koch, D. F. A. In Electrochemistry; Friend, J. A., Gutmann, F., Eds.; Pergamon Press: New York, 1965; p 657. (18) Hunger, H. F. J. Electrochem. Soc. 1969, 116, 1519. (19) Xu, Y.; Schell, M. J. Phys. Chem. 1990, 94, 7137. (20) Okamoto, H.; Tanaka, N. Electrochim. Acta 1993, 38, 503. (21) Willsau, J.; Heitbaum, J. J. Electroanal. Chem. 1985, 185, 181. (22) Capon, A.; Parsons, R. J. Electroanal. Chem. 1973, 45, 205. (23) Kazarinov, V. E.; Vassil’ev, Y. B.; Andreev, V. N.; Kuliev, S. A. J. Electroanal. Chem. 1981, 123, 345. (24) Wilhelm, S.; Iwasita, T.; Vielstich, W. J. Electroanal. Chem. 1987, 238, 383. (25) Beden, B.; Bewick, A.; Lamy, C. J. Electroanal. Chem. 1983, 148, 147. (26) Solomun, T. Surf. Sci. 1986, 176, 593. (27) Beden, B.; Lamy, C.; Bewick, A.; Kunimatsu, K. J. Electroanal. Chem. 1981, 121, 343. (28) Miki, A.; Ye, S.; Osawa, M. Chem. Commun. 2002, 1500. (29) Miki, A.; Ye, S.; Osawa, M. J. Electroanal. Chem. 2004, 563, 23. (30) Chen, Y. X.; Miki, A.; Ye, S.; Sakai, H.; Osawa, M. J. Am. Chem. Soc. 2003, 125, 3680. (31) Parsons, R.; VanderNoot, T. J. Electroanal. Chem. 1988, 257, 9. (32) Okamoto, H.; Tanaka, N.; Naito, M. J. Phys. Chem. A 1997, 101, 8480. (33) Okamoto, H.; Kon, W.; Mukouyama Y. J. Phys. Chem. B 2004, 108, 4432. (34) Capon, A.; Parsons, R. J. Electroanal. Chem. 1973, 44, 239. (35) Kita, H.; Katagiri, T. J. Electroanal. Chem. 1987, 220, 125.

Five Current Peaks in Voltammograms for Oxidations of Formic Acid

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